Extract blocks of numbers from array in Matlab - arrays

Question

I have a vector and I would like to extract all the blocks from it:
x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4]
so that I will get vectors or a cell containing the blocks:
[1 1 1], [4 4], [5 5], [4], [6], [1], [2], [4 4 4], [9], [8], [4 4 4 4]
Is there an efficient way to do it without using for loops? Thanks!

Answer 1

You can use accumarray with a custom anonymous function:
y = accumarray(cumsum([true; diff(x(:))~=0]), x(:), [], #(x) {x.'}).';
This gives a cell array of vectors. In your example,
x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4];
the result is
y{1} =
1 1 1
y{2} =
4 4
y{3} =
5 5
y{4} =
4
y{5} =
6
y{6} =
1
y{7} =
2
y{8} =
4 4 4
y{9} =
9
y{10} =
8
y{11} =
4 4 4 4

Answer 2

For loops aint as slow as you might think, especially not in more recent Matlab versions and especially not in our case. Maybe this will help
x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4];
breakIdx = [0, find(diff(x)), length(x)];
groups = mat2cell(x,1,diff(breakIdx));
We find the groups by applying diff(x) and we get the group indices with find(). Then it's just a matter of moving the groups into the resulting cell groups.
There's very little edge case checks here so I recommend you add that.

Answer 3

If holding all blocks in a cell array is not so important, but ruther the full information about them, you can use this code:
x = [1 1 1 4 4 5 5 4 6 1 2 4 4 4 9 8 4 4 4 4];
elements = x(diff([0 x])~=0);
block_size = accumarray(cumsum(diff([0 x])~=0).',1).';
blocks = [elements; block_size];
to get a 2-row matrix with the element on the first row, and the block size on the second:
blocks =
1 4 5 4 6 1 2 4 9 8 4
3 2 2 1 1 1 1 3 1 1 4
Then define a function to create those blocks by need:
getBlock = #(k) ones(1,blocks(2,k))*blocks(1,k);
and call it with the number of block you want:
getBlock(8)
to get:
ans =
4 4 4